A lot of officers used to wear swivel holsters because they are more comfortable while driving/riding in cars. The holster will swing up so that it doesn't dig into the seat and push up on the duty belt (especially with a low-ride holster). The problem with the older holsters was that they locked in the carry position with a couple of snaps, and if the officer forgot to fasten the snaps, the holster was not locked into place.
When mid and high-rise security holsters became more popular, they reduced the demand for swivel holsters. The holsters with solid belt loops are much more secure, and the gun is always at the right place when you reach for it. Swivel holsters, if they are not locked into position properly, have a tendency to flop around when the officer runs or fights. Reaching for your weapon and finding that it isn't where it is supposed to be is not a good thing.....